Water treatment system

ABSTRACT

Treatment of halogenated hydrocarbon contaminants in groundwater is accomplished by passing the water through a bed of granular iron (43). An electrical circuit (47) is promoted for providing electrons for reducing the contaminant. The circuit may be made using a DC supply, by configuring an electrolytic circuit, or by providing a layer of a second metal such as zinc placed next to the iron bed, thereby creating a galvenic circuit.

This invention relates to the treatment of water, especiallygroundwater, contaminated with halogenated hydrocarbons, such as carbontetrachloride. Such contaminants can be difficult to treat ingroundwater, because their natural degradation rate is very slow, theyare transported long distances through the aquifer with the groundwater,and they are hazardous even in very small trace concentrations if theyget into drinking water supplies.

BACKGROUND TO THE INVENTION

Patent publication WO-91/08176 disclosed the technique of passing watercontaminated with an halogenated hydrocarbon through a (permeable) bodyof a metal, for example through a body of granular iron. The body ofgranular iron was placed in a trench excavated in the ground in the pathof an oncoming plume of the contaminated groundwater, whereby thegroundwater was caused to pass through the metal. Or, the contaminatedwater was taken out of the ground, and passed through a container ofgranular iron.

Developments of that technology are disclosed in WO-92/19556 and inWO-92/19545.

Provided there is a substantial residence time, and provided thatstrictly reducing conditions can obtain over a prolonged period, tracesof halogenated hydrocarbons in the water can be caused to break downchemically in the presence of the iron or other metal.

It is surmised that the chemical breakdown reaction may be explained asfollows:

Under the conditions of the process, iron metal oxidizes to the ferrousion, releasing two electrons, i.e.

Fe←→Fe2++2e-

The halogenated hydrocarbon may be regarded as comprising acarbon-halogen component, C-Hal, and a hydrogen ion. Upon interactingwith the electrons, the carbon reacts with the hydrogen ion to form arelatively non-hazardous hydrocarbon, such as methane gas, and halogenion in solution, e.g chloride etc.

H++C-Hal+2e-←→C-H+Hal-

Thus, the halogenated hydrocarbon breaks down in the presence of iron,under reducing conditions.

However, what can also happen is that electrons available from the ironcould, under reducing conditions, cause the surrounding water todissociate, i.e

2e-+2H₂ O←→H₂ +2OH-

The H₂ bubbles off as hydrogen gas, but the presence of the 2OH- servesto raise the pH of the water, which can rise high enough, say to 9 or10, that dissolved inorganic species present in the water, whichprecipitate out of solution at high pH, can start to do so. At high pH,for example, carbonates of various kinds, which are nearly alwayspresent in groundwater, can precipitate.

Under high pH conditions (i.e a pH of 9 or 10) the ferrous ions, ifplentiful, could combine with the dissolved substances, and ferrouscarbonate or ferrous hydroxide may precipitate.

The precipitates are a problem for the process of decomposition ofhalogenated hydrocarbons because they tend to become deposited in thepore spaces of the body of granular iron, and to clog up the body,making the body not so permeable to the flow of groundwater.

The invention is aimed generally at promoting the breakdown of thehalogenated hydrocarbon. The invention, in a particular aspect, is aimedat inhibiting the precipitation of the iron species and other substancesfrom solution, which, if permitted, might reduce the permeability of thebody of granular iron, and might coat the particles of iron withsubstances that would impede the reduction of the halogenatedcontaminant.

GENERAL FEATURES OF THE INVENTION

The invention lies in providing a body of a first metal, for exampleiron, the metal being in finely divided powder, particulate, or granularform, and the body being porous and permeable enough for the water topass therethrough.

Conditions should be maintained whereby oxygen and oxidising agents areexcluded from the body of metal and from the water. One preferred mannerin which oxygen may be excluded lies in the fact that the process iscarried out below the water table.

It may be noted that if there is any oxygen present in the water, thatoxygen will have to be removed from the water before the breakdownreaction will start. If only small quantities of oxygen are present,that is not very important, because the oxygen will usually be quicklyused up in oxidizing small quantities of the metal. When all the oxygenthat was dissolved in, or was otherwise available in, the water has beenused up, the breakdown reaction may be expected to commence. Largequantities of dissolved oxygen would be a problem in the invention,however, because then much of the metal would simply be wasted throughbeing oxidised, and because the reducing conditions required for thebreakdown reaction would not be obtainable.

The invention lies in setting up an electrochemical circuit, by makingthe first metal an electrode, and maintaining the first metal at apotential relative to the surrounding water.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

By way of further explanation of the invention, exemplary embodiments ofthe invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic view of a system for treating contaminatedwater in a canister or tank;

FIG. 2 is a diagrammatic view of a system for treating contaminatedgroundwater in-situ;

FIG. 3 is a diagrammatic view of another system for treatingcontaminated groundwater in-situ;

FIG. 4 is a diagrammatic view of a further system for treatingcontaminated groundwater in-situ.

The apparatuses shown in the accompanying drawings and described beloware examples which embody the invention. It should be noted that thescope of the invention is defined by the accompanying claims, and notnecessarily by specific features of exemplary embodiments.

In FIG. 1, a tank or container 20 is provided, which in this case ismade of an electrically-non-conducting material, being a plasticmaterial.

Into the tank 20 is placed a body 23 of treatment material, whichcomprises a body of granular iron. The body of granular iron is soconstituted that the body is porous or permeable to the passage of watertherethrough.

The tank 20 is provided with inlet 25 and outlet 27 pipes, forconducting water through the tank. The body of granular iron completelyfills the cross-sectional shape of the tank, whereby water cannot passthrough the tank from the inlet to the outlet other than by passingthrough the granular iron.

Electrodes 29,30 are placed at the ends of the tank, close to the inletand the outlet respectively. The electrodes should be of stainless steelor other non-corrosive conductor. The electrodes are coupled to a DCbattery 32, whereby a voltage and current are applied between the twoelectrodes.

The body 23 of granular iron is so set up in the tank 20 that theelectrodes 29,30 are in electrical contact with the opposite ends of thebody of granular iron, whereby the voltage and current are applied tothe body.

In use, water contaminated with an halogenated hydrocarbon is fedthrough the tank 20. The presence of the voltage and current has beenobserved to increase the rate at which the halogenated hydrocarbonbreaks down; and also has been found to inhibit the deposition of ironand other precipitates.

In a particular example, a column 10 cm in length and 8 cm in diameterwas packed with granular iron having a mean grain diameter of 100 mesh.Water containing 10 milligrams per litre of tetrachloroethene (PCE) insolution was passed through the column. About 2 volts DC was appliedacross the stainless steel electrodes, resulting in a flow of electronsthrough the column.

It was observed that the rate of degradation of the PCE (ie theconcentration of PCE in the outlet compared with the concentration ofPCE in the inlet) speeded up by a factor of about three times. Fromthis, it may be surmised that the electric current acts as an additionalsource of electrons (additional, that is, to the electrons arising fromthe oxidation of the iron), resulting in increased rates of removal ofthe contaminants.

FIG. 2 shows an arrangement suitable for in-situ (i.e in-ground)operation, for treating contaminated groundwater while the water remainsin the ground.

Here, a trench 40 is excavated in the material of the aquifer, and thebody 43 of granular iron is placed in the trench. The body 43 maycomprise granular iron mixed with sand, as an inert filler, for bulk,(so long as adequate electrical conductivity was maintained in thebody), or granular iron mixed with an adsorbing agent, such as activatedcarbon. As mentioned in WO-92/19556, the adsorbent retards the dissolvedcontaminants while the water passes through un-retarded, whereby thecontaminants are retained in the trench (and close to the granular iron)for a much longer period of time than the water itself stays in thetrench. Providing the adsorbent maximises the likelihood that there istime for the breakdown reaction to be completed, while minimising theneeded quantity of granular iron.

In FIG. 2, electrodes, comprising rods 45 of stainless steel, areinserted in the body 43 of granular iron. The electrodes are coupled toa DC battery 47, and the other side of the battery is grounded, as at49. Thus, the electrical circuit is completed through the material ofthe aquifer and through the groundwater.

It might be considered from the diagrams that in FIGS. 1 and 2 the wholebody of granular iron constitutes the one electrode.

The iron itself, of course, being a metal, and conductive, will notsupport very much of a voltage gradient.

It might be surmised that, insofar as the body 43 of iron is itself anelectrode, that the body of iron should be the anode, whereby theoxidation of the iron would be enhanced, releasing more electrons.

However, it is observed that the breakdown of the halogenatedhydrocarbon is enhanced when the iron is made the cathode, and it isobserved that the breakdown proceeds at a faster rate. Also, it isobserved, when the iron is made the cathode, that there is lessdeposition of precipitated material present in the iron bed.

It may be surmised that, in order for the halogen breakdown to bespeeded up, the conditions must have been favouring reduction of thehalogen in the iron bed, at the expense of oxidation of the iron, whileoxygen evolution was happening at the anode.

In the iron bed, the iron oxidation reaction is:

Fe→Fe2++2e-

This is the reaction that is slowed by the fact that the iron bed is thecathode.

At the anode, the oxygen evolution reaction, due to the electrolysis ofthe water, proceeds as

2H₂ O→O₂ +4H++4e-

whereby the H+, upon entering solution, also results in a lowering ofthe pH.

Making the iron bed the cathode results in the favouring of reduction ofthe halogen contaminant at the expense of iron oxidation, while oxygenevolves at the anode. The pH is lowered, at least near the anode, andprecipitation of iron compounds is lowered because of the lower ironoxidation.

In order for the reduction of the halogenated hydrocarbon to proceed,the conditions must remain reducing, and the system designer shouldensure that reducing conditions are favoured. This may be done, in anin-ground treatment system, for instance, by providing that all the ironis placed well below the water table. Also, for instance, the oxygen gasthat bubbles off the anode due to electrolysis of the water must not beallowed to come in contact with or pass through the iron bed. In atreatment installation, the system designer should see to it that theanode is so located that oxygen bubbling therefrom will not pass throughthe bed of iron.

The anode may be configured as a separate series of stainless steel rodsinserted in the aquifer material, downstream of the iron bed. Any oxygenbubbling off the anode then would not affect the iron bed.

In considering why the halogen reduction proceeds quicker when the ironbed is made the cathode, it may be noted that in fact the number ofelectrons produced by the naturally-oxidizing iron is already ample.Therefore, the body of iron may be made the cathode. Although thisinhibits oxidation of the iron, the benefit of making the iron thecathode is that less of the iron precipitates.

It should be pointed out that the reduction of the halogenatedhydrocarbon, by the use of electricity, requires that the electrode(preferably the cathode) at which the reduction takes place is of alarge surface area. The granular iron provides such a large-areaelectrode.

FIG. 3 shows an in-ground water treatment installation, in which atrench 50 is excavated in the path of an on-coming plume 52 of agroundwater contaminant, being a halogenated hydrocarbon such as carbontetrachloride. In the trench 50 is placed a body 54 of granular iron.

Located on top of the body 54 of granular iron is a layer 56 of zincfilings. In place of zinc, another metal may be used having a lowerpotential than iron. The zinc filings are placed directly on top of thegranular iron, whereby electrical contact is made between the twometals.

The zinc is placed below the water table 58, and the space 60 above thewater table is filled in with sand, or other available filler material.

No outside source of electricity is connected to the metals, but in factthe metals themselves in this configuration serve to generate anelectric current. In an iron/zinc pair, the iron becomes the cathode andthe zinc the anode.

The zinc oxidizes, Zn→Zn2++2e-, which creates the supply of electronsneeded for the breakdown of the contaminant. The iron is not called uponto oxidize, and to supply electrons for the reduction of thecontaminant. There may be some deposition of precipitates, such as zinccarbonate, in the zinc layer, but that does not matter so much,especially if the treatment system is arranged so that the water, ormost of the water, is not actually required to pass through the zinclayer. The main bulk of material on which the breakdown of thecontaminant occurs remains the iron. Iron is considerably cheaper thanzinc in bulk quantities, and besides, iron is relatively harmless if itshould become dissolved in the groundwater--if forced to choose, mostauthorities would prefer to have Fe2+ in the treated groundwater ratherthan Zn2+.

It may be noted that mixing or dispersing the zinc in with the iron hasless effect in promoting the breakdown of the halogenated hydrocarbon.The zinc should preferably be placed in a layer, as described, wherebythe layer of zinc can act as a separate electrode with respect to thebed of iron. However, it is also contemplated that the zinc may beplaced in two or more layers disposed through the iron.

The zinc may be placed on top of the bed of iron, as shown; or, the zincmay be placed in series with the bed of iron in the ground. Patentpublication WO-93/22241 shows a funnel-and-gates treatment system; inthat system, contaminated water is funnelled through a gate, in which isplaced the treatment material. In FIG. 4, a barrier 60 is placed in theground, and the moving groundwater is funnelled into a gate 63. In thegate is placed a bed of granular iron 65. Behind the bed of iron (i.e inthe gate, but downstream of the iron) is placed a bed of zinc 67. Thezinc and the iron are arranged so as to promote the electrochemicalactivity as described. The zinc should be so placed in relation to theiron that the electrons arising from the oxidation of the zinc aredonated to the iron.

The zinc, being a metal of lower potential than the iron, and arrangedas an anode, donates electrons by galvanic action. Applying suppliedelectricity to the first metal as one electrode, the other electrodebeing separated, the electrons are provided by electrolytic action. Asdescribed, both the galvanic and the electrolytic actions may beutilized to enhance the breakdown of halogenated hydrocarbons ingroundwater.

Although the invention has been described as it relates to treatmentusing iron, other metals may be used, for example: zinc, aluminum,magnesium, other transition metals, and metal couples such as coppercoated iron.

The invention may be used to treat a wide range of organic contaminants,including aliphatics, aromatics, and polyaromatics with halogen andnitrogen group substituents (although the invention has been describedas it relates to the treatment of halogenated hydrocarbons). Examplesinclude solvents such as carbon tetrachloride, tetrachlorethene, andhexachlorethane; hexachlorobenzene, nitrosamines, explosives such astrinitrotoluene, PCP's, nitro-PAH's, and certain pesticides.

We claim:
 1. Procedure for treating water contaminated with an organiccontaminant, comprising the steps of;providing a body of a metal, themetal being in finely divided particulate form, and the body beingporous and permeable enough for the water to pass therethrough; passingthe contaminated water therethrough; creating and maintaining reducingconditions in the body of the metal and in the water passingtherethrough; creating an electrolytic circuit, comprising twoelectrodes, being an anode and a cathode, and an electrolyte;configuring the body of the metal as one of the electrodes, and thecontaminated water as the electrolyte; providing a source of electricalenergy, and applying a sufficient voltage derived from the said sourcebetween the two electrodes, and maintaining the body of metal at avoltage relative to the passing contaminated water to enhance breakdownof the organic contaminant in the water.
 2. Procedure of claim 1,wherein the contaminant is a halogenated hydrocarbon.
 3. Procedure ofclaim 1, wherein the metal is iron.
 4. Procedure of claim 1, includingthe step of so configuring the electrolytic circuit that the body of themetal is a cathode in the circuit.
 5. Procedure of claim 1, includingthe steps of:placing the body of metal in an aquifer, or in the ground;wherein the contaminated water is groundwater in the aquifer; soarranging the body of metal in the aquifer that the contaminatedgroundwater passes therethrough; and configuring the aquifer or groundmaterial in which the body of metal is placed as the second electrode,and the groundwater as the electrolyte.
 6. Procedure of claim 5,including the step of locating the body of metal substantially whollybelow the water table.
 7. Procedure of claim 5, including the step ofapplying the voltage to the body of the metal through a conductor, whichcomprises a metal rod or rods inserted into the body of the metal. 8.Procedure of claim 5, including the step of applying the voltage suchthat the body of the metal is the cathode of the electrolytic circuit.9. Procedure of claim 8, including the step of configuring the anode ofthe electrolytic circuit as a separate series of metal rods, inserted inthe aquifer material, downstream of the body of the metal.
 10. Procedureof claim 8, including the step of so placing the anode in relation tothe body of the metal that gases produced at the anode by theelectrolytic action can bubble off the anode and can escape anddissipate without contacting the body of the metal.
 11. Procedure fortreating water contaminated with an organic contaminant, comprising thesteps of;providing a body of a first metal, the metal being in finelydivided particulate form, and the body being porous and permeable enoughfor the water to pass therethrough; passing the contaminated watertherethrough; creating and maintaining reducing conditions in the bodyof the first metal and in the water passing therethrough; providing anelectrochemical circuit, by making the body of the first metal anelectrode, and maintaining the body of the first metal at a voltagerelative to the surrounding water; placing the body of the first metalin an aquifer, or in the ground, the contaminated water beinggroundwater in the aquifer; so arranging the body of the first metal inthe aquifer that the contaminated groundwater passes therethrough;creating the electrochemical circuit as an electrolytic circuit,comprising two electrodes, being an anode and a cathode, and anelectrolyte; configuring the body of the first metal as one electrode,and surrounding aquifer or ground material as the second electrode, andthe groundwater as the electrolyte; and applying a sufficient voltagebetween the two electrodes to enhance breakdown of the organiccontaminant in the groundwater.
 12. Procedure of claim 11, including thestep of applying the voltage to the body of the first metal through aconductor, which comprises a metal rod or rods inserted into the body.13. Procedure of claim 11, including the step of applying the voltagesuch that the body of the first metal is the cathode of the electrolyticcircuit.
 14. Procedure for treating water contaminated with an organiccontaminant, comprising the steps of:providing a body of a first metal,the metal being in finely divided particulate form, and the body beingporous and permeable enough for the water to pass therethrough; passingthe contaminated water therethrough; creating and maintaining reducingconditions in the body of the first metal and in the water passingtherethrough; providing an electrochemical circuit, by making the firstmetal an electrode, and maintaining the first metal at a sufficientvoltage relative to the surrounding water to enhance breakdown of theorganic contaminant in the water; placing the body of the first metal inan aquifer, or in the ground; wherein the contaminated water isgroundwater, in the aquifer; so arranging the body of the first metal inthe aquifer that the contaminated groundwater passes therethrough;creating the electrochemical circuit as a galvanic circuit, comprisingtwo electrodes, being an anode and a cathode, in electrical contact witheach other; providing a body of a second metal, and placing the body ofthe second metal in electrical contact with the body of the first metal;so configuring the galvanic circuit that one of the bodies of metal isthe anode, and the other body of metal is the cathode, of the galvaniccircuit; providing the second metal in the form of a metal that is moreelectro-positive than the first metal, whereby the second metal becomesthe anode, and the first metal the cathode, of the galvanic circuit;arranging the two bodies as separate structures or layers, which arearranged for electrical contact with each other, but are so arrangedthat the two metals are substantially not mixed together; physicallyarranging the bodies so that a majority of the contaminated groundwaterpasses through the first metal, but does not pass through the secondmetal.
 15. Procedure of claim 14, including the step of:placing the bodyof the first metal in a trench, the trench being located in the path ofan oncoming plume of the contaminated groundwater; placing the body ofthe second metal in the trench, and on top of the body of the firstmetal; and so arranging the trench and the bodies therein that the bodyof the second metal lies below the water table.
 16. Procedure of claim14, wherein the second metal is zinc.